PROJECT SUMMARY People with Parkinson disease (PD) experience various motor impairments, including locomotor deficits that may reduce their mobility and quality of life. In particular, the majority of people with PD report difficulties with turning in place or turning while walking. Problems with turning may lead to falls and serious injuries, such as hip fractures. Hip fractures are about eight times more likely to occur when an individual falls during a turn, compared to during straight walking. Relatively little is known about the mechanisms of turning difficulties and there are currently no effective treatments that specifically target turning difficulties. This study will help elucidate the mechanisms of turning difficulty and assess a potential novel treatment option for turning difficulties. Turning difficulties in PD may occur as a result of impairments in an individual's ability to select and execute the appropriate motor patterns to complete a turn, which may be improved by motor training. It is suspected that turning difficulties are only minimally improved by prescribed anti-parkinson medications, which would indicate lack of a substantial contribution of dopaminergic systems to turning impairments. External cues, on the other hand, have been shown to be relatively effective for improving motor function in people with PD. Stepping on a rotating treadmill may serve as an external cue to facilitate turning, resulting in measurable changes in how turns are completed. Completing training sessions on the rotating treadmill may reinforce the appropriate turning motor patterns or make these correct patterns more automatic, resulting in functional improvements in turning. This study will assess turn kinematics, muscle activity patterns during turning, and functional measures of turning to determine whether: 1) anti-parkinson medications improve turn performance in PD, 2) rotating treadmill stimulation results in improved turn performance in PD, 3) greater changes in turning performance are seen following multiple rotating treadmill training sessions, and 4) changes in turning performance are maintained following rotating treadmill stimulation, making the treadmill a viable option for rehabilitation applications. Major goals of the research proposed include achieving a better understanding of the mechanisms of turning difficulty, and also learning more about the locomotor adaptations that occur following rotating treadmill training. Insight gained from these studies will help in the development of innovative methods for the rehabilitation of gait and turning impairments in people with PD, which will potentially reduce the incidence of falls and injury in these individuals.